1. Technical Field
The present invention relates to a circuit for measuring ionization current in a combustion chamber of an internal combustion engine.
2. Discussion
An internal combustion engine produces power by compressing a fuel gas mixed with air in a combustion chamber with a piston and then igniting the mixed gas with an ignition or spark plug. When combustion of the mixed gas occurs in the combustion chamber, the gas is ionized. If, after combustion, a bias voltage is applied between the ignition plug electrodes, then an electric current is produced which passes through the chamber due to the ions generated during the combustion process. This electric current is commonly referred to as ionization current. Since the ionization current varies with respect to the characteristics of the combustion, measurement of the ionization current provides important diagnostic information regarding engine combustion performance.
Several circuits have been proposed for detecting ionization current, however these prior art detection circuits have several shortcomings. In prior art detection circuits, the ignition current (which is produced in response to the combustion of the mixed gas) and the ionization current flow in opposite directions through the secondary winding of the ignition coil, thus requiring the ionization current to overcome the stored energy in the secondary winding of the ignition coil before the ionization current can be detected. As a result, the initiation or, in other words, the flow of ionization current as well as the detection of ionization current is delayed in time. Further, in prior art detection circuits, the ionization current is detected by way of a current mirror circuit which requires a second power source other than the ignition coil. Typically, the second power source supplies a relatively low voltage (e.g. 1.4 volts) to the current mirror circuit. As a result, the magnitude of the mirrored current signal is relatively small and the signal-to-noise ratio is low. Even further, prior art detection circuit designs are complex and, therefore, costly. Accordingly, there is a desire to provide a circuit for measuring ionization current which overcomes the shortcomings of the prior art.